in a car, you give it more gas to make it go faster. in a diesel, you give it more air.
diesels inject the EXACT same amount of fuel in each injection stroke. you feed it more air, you get better burn. more burn, more power, better efficiency.
so, freeing up the restrictions on the intake side lets you move more air.
additionally, by drawing air from outside the grill support, it is 'colder' than the air it would have sucked from the backside.
Be sure to relay this wisdom to the performance injector companies.